DNA vaccines

Use of a mouse lung challenge model to identify antigens protective against Chlamydia pneumoniae lung infection

Chlamydia pneumoniae is emerging as a significant human pathogen. Infection causes a range of respiratory tract diseases and is associated with atherosclerosis. A vaccine could provide a considerable public health benefit; however, antigens able to elicit a protective immune response are largely unknown. A panel of open-reading frames (ORFs) from the C. pneumoniae genome sequence was screened for ability to elicit protective responses. Balb/c mice immunized with DNA containing the ORFs were tested for their ability to limit lung infection following an intranasal challenge. Immunization with DNA encoding the major outer membrane protein or an ADP/ATP translocase (Npt1(Cp)) of C. pneumoniae resulted in a reduced bacteria load in the lung after challenge. The identification of these antigens as protective is a significant step toward development of a C. pneumoniae vaccine and demonstrates the feasibility of using a DNA immunization strategy to screen the C. pneumoniae genome for other protective ORFs.

New approaches for imaging in gene therapy

Gene therapy is increasingly used experimentally and clinically to replace defective genes and/or impart new functions to cells and tissues. With the recent advances in vector design, improvements in transgene and prodrug activation strategies, gene therapy has been applied to a wide variety of diseases, tissues and organ systems. It is now clear that our specialty will play a critical role in gene therapy research and its clinical applications. Three aspects of gene therapy are of particular interest to imaging. The first is in delivering genes and vector products by minimally invasive interventional techniques. The second is in quantitating gene and DNA deliveries, for example, by nuclear imaging. Finally, imaging can be used to monitor the levels of transgene expression in vivo. A variety of imaging techniques including PET imaging, nuclear imaging, MR imaging and optical imaging can potentially be used to achieve the latter. This brief introductory overview is intended to summarize current strategies and illustrate the role that radiology will play in this field.

DNA vaccination against influenza viruses: a review with emphasis on equine and swine influenza

The influenza virus vaccines that are commercially-available for humans, horses and pigs in the United States are inactivated, whole-virus or subunit vaccines. While these vaccines may decrease the incidence and severity of clinical disease, they do not consistently provide complete protection from virus infection. DNA vaccines are a novel alternative to conventional vaccination strategies, and offer many of the potential benefits of live virus vaccines without their risks. In particular, because immunogens are synthesized de novo within DNA transfected cells, antigen can be presented by MHC class I and II molecules, resulting in stimulation of both humoral and cellular immune responses. Influenza virus has been used extensively as a model pathogen in DNA vaccine studies in mice, chickens, ferrets, pigs, horses and non-human primates, and clinical trials of DNA-based influenza virus vaccines are underway in humans. Our studies have focused on gene gun delivery of DNA vaccines against equine and swine influenza viruses in mice, ponies and pigs, including studies employing co-administration of interleukin-6 DNA as an approach for modulating and adjuvanting influenza virus hemagglutinin-specific immune responses. The results indicate that gene gun administration of plasmids encoding hemagglutinin genes from influenza viruses is an effective method for priming and/or inducing virus-specific immune responses, and for providing partial to complete protection from challenge infection in mice, horses and pigs. In addition, studies of interleukin-6 DNA co-administration in mice clearly demonstrate the potential for this approach to enhance vaccine efficacy and protection.

Cytotoxic T-lymphocyte responses elicited to Wilms' tumor gene WT1 product by DNA vaccination

We recently have reported that Wilms' tumor gene WT1 is highly expressed not only in leukemias but also in various types of solid tumors and that WT1 protein is a novel tumor antigen against which cytotoxic T lymphocytes (CTLs) can be elicited by immunization with 9-mer WT1 peptides capable of binding to major histocompatibility complex (MHC) class I molecules. In the present study, plasmid DNA encoding murine full-length WT1 protein was injected intramuscularly into C57BL/6 mice. The mice vaccinated with the WT1 plasmid DNA elicited CTLs against the WT1 protein, and the CTLs specifically killed WT1-expressing tumor cells in a MHC class I-restricted manner. Furthermore, the vaccinated mice rejected the challenges of WT1-expressing tumor cells and survived with no signs of autoimmunity caused by the CTLs. These results demonstrated that vaccination with the WT1 plasmid DNA can elicit CTL responses specific for the WT1 protein, resulting in the acquisition of rejection activity against challenges of WT1-expressing tumor cells. This WT1 DNA vaccination may find clinical application for various types of solid tumors as well as leukemias.

Oral delivery of DNA vaccines using attenuated Salmonella typhimurium as carrier

The efficacious delivery of eukaryotic expression plasmids to inductive cells of the immune system constitutes a key prerequisite for the generation of effective DNA vaccines. Here, we have explored the use of bacteria as vehicles to orally deliver expression plasmids. Attenuated Salmonella typhimurium aroA harbouring eukaryotic expression plasmids that encoded virulence factors of Listeria monocytogenes were administered orally to BALB/c mice. Strong cytotoxic and helper T cell responses as well as antibody production were elicited even after a single administration. Mice immunised four times with Salmonella that carried a eukaryotic expression plasmid encoding the secretory listerial protein listeriolysin were protected against a subsequent lethal challenge with this pathogen. A single dose was already partially protective. The efficiency of this vaccination procedure was due to transfer of the expression plasmid from the bacterial carrier to the mammalian host. Evidence for such an event could be obtained in vivo and in vitro. Expression of the desired antigen in various lymphoid tissues was already detectable 1 day after administration of the DNA vaccine and persisted for at least 1 month in spleen and mesenteric lymph nodes. Induction of cytotoxic and helper T cell responses was observed in all mouse strains tested including outbred strains whereas antibodies were mainly detected in BALB/c. Furthermore, we could show that immunogenicity could be improved by increasing the invasiveness of the bacterial carrier.

Modulation of immune responses following antigen administration by mucosal route

Most microbial infections are either restricted to the mucosal membranes or the etiologic agents needed to transit the mucosa. Thus, it is desirable to stimulate a mucosal response following vaccination, to block both infection and disease development. Attenuated vaccine carriers mimic natural infections, triggering also mucosal responses. Similar results can be achieved by administering antigens with appropriate adjuvants. However, the delivery of antigens per se is not sufficient to engender a protective response. A successful immunization requires the elicitation of an appropriate type of immune response (e.g. antibodies vs. cell-mediated immunity, Th1 vs. Th2 helper pattern). Therefore, a successful vaccination strategy demands the choice of adequate antigens, and their appropriate delivery and/or formulation to promote the required quality of immune response. Different strategies to optimize the immune responses elicited following vaccine administration by the mucosal route are discussed.

A DNA Immunization Model Study with Constructs Expressing the Tick-Borne Encephalitis Virus Envelope Protein E in Different Physical Forms

We have conducted a DNA immunization study to evaluate how the immune response is influenced by the physical structure and secretion of the expressed Ag. For this purpose, we used a series of plasmid constructs encoding different forms of the envelope glycoprotein E of the flavivirus tick-borne encephalitis virus. These included a secreted recombinant subviral particle, a secreted carboxyl-terminally truncated soluble homodimer, a nonsecreted full-length form, and an inefficiently secreted truncated form. Mice were immunized using both i.m. injection and Gene Gun-mediated application of plasmids. The functional immune response was evaluated by determining specific neutralizing and hemagglutination-inhibiting Ab activities and by challenging the mice with a lethal dose of the virus. As a measure for the induction of a Th1 and/or Th2 response, we determined specific IgG subclasses and examined IFN-γ, Il-4, and Il-5 induction. The plasmid construct encoding a secreted subviral particle, which carries multiple copies of the protective Ag on its surface, was superior to the other constructs in terms of extent and functionality of the Ab response as well as protection against virus challenge. As expected, the type of Th response was largely dependent on the mode of application (i.m. vs Gene Gun), but our data show that it was also strongly influenced by the properties of the Ag. Most significantly, the plasmid encoding the particulate form was able to partially overcome the Th2 bias imposed by the Gene Gun, resulting in a balanced Th1/Th2 response.

DNA vaccines for cancer therapy

Vaccination with a tumour antigen-expressing plasmid DNA (pDNA) is a novel approach to human cancer immunotherapy. Initial results in preclinical rodent tumour models are promising, revealing that pDNA cancer vaccines can elicit both humoral, as well as cell-mediated immunity and, in some cases, protect against tumour growth. Compared to peptide, viral or dendritic cell vaccines, the delivery of tumour antigens using pDNA has the advantages of ease of manufacture, lack of toxicity and broad applicability to large populations. With advances in modern genomics strategies and the identification of an increasing number of tumour antigen genes, pDNA-based cancer vaccines may be used in the future to treat a wide variety of human cancers.